Not many guys....or girls, post until there is a problem but it's a good question to start a positive thread for a change.
After I bought my M1010 she plowed snow and ran parades for 6000 miles with just regular maintenance. I did baby it, bathed her, parked her in a warm shop, so the driving conditions and driver will dictate life-span. My problems came when it sat for a year and a half. Having a great knowledge base on what 'not to do'...like trying to start them with one bad battery....helps extend the life. I expect that the new owner will be able to run it for another 20 years if he treats her good.
My 60 year old M135 has toured for 5 years with a few minor leaks but with a serious maintenance regime she just keeps going. Restoration was done on Brakes, Steering, Rubber, electrical, Paint, etc but it still has all the original drive-train that I bought it with.
It'll be cool reading about other long term life-spans the honored Steel Soldiers have achieved. Great firstish post and welcome aboard to Steel Soldiers!